Running the Glass Dewar

Jared Mehl 3-3-03

 

  1. Start evacuation of spectrometer and turn on lamp with cooling water first, allowing system plenty of time for system to thermally stabilize before measurement.
  2. Once you’ve loaded all the appropriate films and filters into your rotator rig, bolt it into the glass dewar for a tight O-ring seal.
  3. To test for leaks in the He pumping line, turn on the basement He pump without any cryogens in system.  The line should pump down to the effective zero of the gauge.  Close the pumping line, and make sure the system remains at vacuum, although a slow leak (few Torr over the course of 15 minutes) is probably OK.  Vent system and turn off basement pump for now.
  4. Next, evacuate the bolometer can inside the rotator rig.  This should be done at room temperature with the turbo-pump.  Unless you are very confident that the bolometer seal is leak tight, you probably want to leave the turbo-pump on for the whole measurement.  You can leak check the bolometer can while in the glass dewar system.
  5. Flush He gas out of vacuum space He dewar and leave air as exchange gas (will freeze out).  To do this, evacuate the He glass dewar with the small mechanical pump located in the back left side under the FTS table.  Use the bleeder valve (on left hand side of FTS table) to release air into the dewar a few times to help pull out any He gas.  Once the pressure has approached a minimum value (between 100 and 200 mTorr is good), seal off the glass tube with knob on top of the dewar support plate.  Use bleeder valve to release air back into mechanical pump, then turn it off.
  6. Fill space between glass dewars with LN2.  Let system cool down for roughly 3 hours.
  7. Once system has cooled down, top off LN2.
  8. Back-fill He dewar with  N2 or He gas.  
  9. Transfer LHe through rotator rig into bowl of He glass dewar until you can see LHe through glass of dewar near top.  Be sure to use a fairly long transfer tube, so that the cold He gas is released near the bottom of the rotator rig, where all the heat capacity is, to ensure an efficient transfer.  Remove transfer tube and plug hole.
  10. Using the basement He pump again, pump on the LHe to reduce its temperature (should get down to roughly 1.3 K).  Use the needle valve until the pressure is half atmospheric pressure, then open the big lever valve slowly, which should bottom out the left pressure gauge at a few Torr.
  11. Slide FTS on top of glass dewar and attach section light pipe between rotator rig and spectrometer.
  12.  Hook up vacuum (clear plastic tube) of FTS chamber to light pipe section, open bleeder valve, and use larger mechanical pump to empty the light pipe and FTS chamber (gets rid of water vapor lines), if not only done at first step.
  13. Check to make sure cooling water is on, then turn on FTS lamp.
  14. Perform measurement.
  15. When finished, detach and shut down all the FTS stuff as you usually would.  Its safe to leave the glass dewar on the building pumping line overnight to let it warm to room temperature gradually.  There are two pressure release valves in the line, so it will be safe overnight.
  16. The next morning, turn off basement He pump and remove rotator rig.